By way of example, a surgical microscope of the type set forth at the outset is used in microsurgery. More specifically, such a surgical microscope can be used in ophthalmic surgery. Within this scope, cataract surgery has become particularly important. A “cataract” is understood to mean the opacification of the natural lens of the eye. In German, this disease is also known as “Grauer Star.” Within the scope of cataract surgery, the opacified, natural lens of the eye is removed from the eye and replaced by an artificial lens for the eye, a so-called intraocular lens. Consequently, an “object” within the meaning of the present disclosure can be an eye of a patient, a specific structure of an eye, for example the cornea, more specifically the vertex of the front side of the cornea, to mention but a few examples.
The cataract surgery is usually carried out by a physician while observing the eye of the patient through an ophthalmic surgical microscope. Within the scope of the cataract operation, a refraction measurement is taken on the phakic, aphakic and/or pseudo-phakic eye. The state of the eye in which the natural lens is present in the eye is understood to be “phakic,” the state in which the natural lens has been removed from the eye and the eye is without lens is understood to be “aphakic,” and “pseudo-phakic” denotes the state in which an artificial lens has been inserted into the eye, wherein the artificial lens may be a temporary lens. The aforementioned refraction measurement comprises a distance measurement between the surgical microscope and the apex of the cornea of the eye, which must be carried out with high accuracy.
By way of example, to be able to measure an aphakic patient's eye with the required measurement accuracy with a wavefront-based intraoperative refraction measurement, the measurement distance must be known, or maintained, within an accuracy range of 0.3 mm.
In general, when using surgical microscopes in certain applications in microsurgery, it is necessary to establish the distance between the surgical microscope and the observed object accurately, which assumes that the object-side focal plane of the surgical microscope is set exactly to the object to be measured.
These days, this setting is realized by the use of autofocus systems, for example, with which surgical microscopes may be equipped. Such autofocus systems can be based on a contrast evaluation of a camera image. To this end, such a surgical microscope comprises a camera which captures the image representation of the object in the image plane. Within the meaning of the present disclosure, a “camera” is understood to mean, in general, an image recorder or an image sensor. In particular, the camera can be a video camera.
The image recorded by the camera is evaluated in terms of the contrast thereof and the autofocus system adjusts the focal plane of the surgical microscope until the camera image of the object recorded by the camera has a maximum contrast. Here, the accuracy of the correct focal plane setting is determined, inter alia, by the depth of field of the imaging of the observed object onto the image plane, in which the camera is situated. Here, “depth of field” is understood to mean a distance range in front of and behind the object-side setting or focal plane, within which an object can be displaced axially without noticeable blurring of the imaging arising in the image plane.
In current surgical microscopes with an autofocus system, an accurate distance measurement cannot be realized, or can only be realized approximately, on account of the depth of field of the imaging that is too high, at least at magnifications of the surgical microscope at which a physician carries out the cataract surgery. By way of example, if the surgical microscope should be set exactly onto the vertex of the front side of the cornea for the purposes of measuring a distance, this cannot be brought about by the autofocus system on the basis of a contrast evaluation of the camera image since regions in front of and behind the vertex of the cornea appear in the camera image with the same high contrast.
On the other hand, it is desirable in principle for surgical microscopes to have a large depth of field because the tissue operated on by the physician does not, as a rule, form a plane but it is craggy instead. Therefore, attempts are always made to facilitate in-focus vision for all regions of an operating region. This is particularly important in the case of surgery on the eye since, by way of the cornea, the pupil and the lens, there are a plurality of transparent tissues that lie over one another.
Thus, U.S. Pat. No. 7,209,293 B2 proposes to further increase the depth of field of a surgical microscope by means of an optical phase-shift element in the imaging beam path of the microscope imaging optical unit. However, this is detrimental to accurately measuring a distance with the surgical microscope. A reduction in the depth of field in the case of autofocus systems for surgical microscopes can be obtained by increasing the aperture of the microscope imaging optical unit. However, this is disadvantageous in that the optical system of the surgical microscope becomes significantly larger overall, as larger free diameters of the microscope imaging optical unit are required.
The phase contrast method for reducing the depth of field is known from the field of digital cameras with a video function; however, the method requires specific image recorders which, for example, have a plurality of sensors in different planes.